A need has recently arisen for a device that converts an optical signal from one frequency to another. This need exists in the field of communication networks utilizing wavelength-division multiplexing (WDM). In such a network, a data signal is impressed upon one optical carrier frequency, and it is desired to thereafter convert the so impressed optical carrier frequency to another optical carrier frequency. Several WDM architectures are, possible. One schematically illustrated in FIG. 1 includes a number of small-area WDM networks 12 interconnected by a long-distance WDM backbone 14. Multiple optical carriers having respective wavelengths .lambda..sub.1, .lambda..sub.2, . . . .lambda..sub.N are impressed on single optical fibers interconnecting respective nodes within each of the small-area networks 12, where N is the number of optical carriers and where the number of nodes is generally of order N. Each node is assigned one optical carrier of wavelength .lambda..sub.i on which to receive signals, but each node can selectively transmit signals on any of the wavelengths .lambda..sub.1, .lambda..sub.2, . . . .lambda..sub.N. As a result, signals can be routed through the small-area networks 12 according simply to their respective carrier wavelengths .lambda..sub.i. This architecture has become advantageous because suitable components have been recently developed which can switch the optical signals according to their optical frequencies without the need for converting them to electrical form.
For a number of reasons including limited bandwidth of optical amplifiers and laser frequency jitter, realistically sized systems will require that the small-area networks 12 and the backbone 14 reuse the same N carrier wavelengths .lambda..sub.1, .lambda..sub.2, . . . .lambda..sub.N, where N is of the order of 10 to 40. Then, in order that any one small-area network 12 be connectable to any other, an optical signal transmitted between a small-area network 12 and the backbone 14 will require a wavelength converter 16 at the interface between them to change the carrier frequency. Such wavelength shifts can be accomplished by demodulating the incoming optical carrier to produce an electrical data signal and then modulating the outgoing optical carrier with the electrical data signal. However, an all-optical wavelength converter not requiring conversion to electrical form is greatly desired. Inoue has disclosed such an all-optical converter using a Mach-Zehnder filter in "Wavelength Conversion for Frequency-Modulated Light Using Optical Modulation to Oscillation Frequency of a DFB Laser Diode," in Journal of Lightwave Technology, vol. 8, 1990, pp. 906-911; but, this converter is bulky and fragile. To date, a practical form of such an all-optical wavelength converter has not been found.